1. Field of the Invention
The present invention relates to a coupling degree adjustment circuit, an antenna device for multiband, and a communication terminal apparatus equipped with the antenna device.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. H06-069715 and Patent Literature Japanese Unexamined Patent Application Publication No. 2003-008326 disclose multiple resonant antennas in which a radiating element and a radiating element are coupled to each other for the purpose of expanding applicable frequency bands. In these multiple resonant antennas, a feeding element and a non-feeding element are parallel to each other in a region in which a magnetic field component is increased and are made magnetically coupled to each other so that each element can act as a radiating element.
The typical configuration of the conventional multiple resonant antennas as disclosed in Japanese Unexamined Patent Application Publication No. H06-069715 and Japanese Unexamined Patent Application Publication No. 2003-008326, as illustrated in FIG. 1A, includes a first radiating element RE1 as a feeding element, and a second radiating element RE2 as a non-feeding element, makes the vicinity of a feeding portion of the first radiating element RE1 and the vicinity of a ground end of the second radiating element RE2 close and in parallel to each other so as to cause the elements to be magnetically coupled to each other.
When a resonant frequency of the first radiating element RE1 is set to f1 and a resonant frequency of the second radiating element RE2 is set to f2, and the first radiating element RE1 and the second radiating element RE2 are coupled to each other, as illustrated in FIG. 1B, the second radiating element RE2 resonates at f2. The return loss characteristic of this whole multiple resonant antenna shows the combination of a resonance characteristic of the first radiating element RE1 and a resonance characteristic of the second radiating element RE2, as illustrated as the solid line of FIG. 1B, and becomes a characteristic as illustrated by a solid line.
However, the strength of coupling between the first radiating element RE1 and the second radiating element RE2 is determined not only by a distance between the elements but under a condition in which the vicinity of a feeding portion of the first radiating element RE1 and the vicinity of a ground end of the second radiating element RE2 are close to each other and also arranged in parallel to each other. Therefore, the flexibility of the pattern of the first radiating element RE1 and the second radiating element RE2 is low. In addition, when the first radiating element RE1 and the second radiating element RE2 are arranged too close to each other, it becomes impossible to match a feeding circuit and the multiple resonant antenna and further, when other components (especially a metal article) exist near a portion in which the elements are parallel (a portion in which the elements are magnetically coupled), a problem that the degree of coupling of the first radiating element RE1 and the second radiating element RE2 may change arises.